A method for producing a GaN laser device using an ScAlMgO4 substrate has been known (see, for example, Patent Literature 1). The lattice mismatch ratio of ScAlMgO4 with respect to GaN (((lattice constant of GaN)−(lattice constant of ScAlMgO4))/(lattice constant of GaN)) is −1.9%, which is smaller than the lattice mismatch ratio of a sapphire substrate thereto (+16%). Accordingly, crystal growth of GaN with an ScAlMgO4 substrate as a seed substrate provides a GaN crystal having a smaller defect density than with a sapphire substrate. Patent Literature 1 describes a method, in which an amorphous or polycrystalline buffer layer is formed on an ScAlMgO4 substrate at a low temperature of approximately 600° C., and then a GaN single crystal thin film is formed by a metal-organic chemical vapor deposition method (which may be hereinafter referred to as an MOCVD method) at a high temperature of 1,050° C.
Patent Literature 2 describes a method, in which a mask is formed on a partial region of a dissimilar substrate, such as a sapphire substrate, different from GaN, and a GaN crystal is grown on the mask selectively in the lateral direction. In the Patent Literature 2, a GaN crystal is grown on a sapphire substrate and an ScAlMgO4 substrate by an ammonothermal lateral epitaxial growth method at a temperature of approximately from 650 to 690° C.
Patent Literature 1: JP-A-2015-178448
Patent Literature 2: JP-A-2014-111527
However, both the techniques of Patent Literatures 1 and 2 have a problem of mismatch in lattice constant, which causes a stress concentration at the interface between the grown crystal and the seed substrate. The stress concentration at the interface may be a factor causing deterioration of the crystal quality, such as an inclination of the crystal axis and occurrence of warpage. Accordingly, there has been a demand of providing a Group III nitride semiconductor containing a Group III nitride crystal having better quality than the ordinary products, and a demand of providing a production method therefor.